Control system for steam refrigerating systems



Nov. s, 1938. A. A. MARKSON 2,136,114

CONTROL SYSTEM FOR STEAM REFRIGERATING SYSTEMS Filed Feb. ll, 1955 INVENTOR Patented Nov. 8, 1938 UNITED STATES PATENT OFFICE CONTROL SYSTEM FOR STEAM REFRIGER- ATING SYSTEMS Alfred A. Markson, Kew Gardens, N. Y., assignor to John M. Hopwood, Dormont, Pa.

Application February 11, 1935, Serial No. 6,053 Claims. (C'l. (i2- 152) 1 evaporator and a condenser into which steam,

Aair, and water vapor are discharged.

In refrigerating systems of this type, whether they be used for air conditioning or for providing a supply of cool water for oflice buildings,

'l5 and the like, the water being cooled which, for

convenience will be called refrigerant herein, is delivered from a source of supply into a flash tank or vacuum chamber in which it is cooled and from which the cooled water iscirculated V$50 to Wherever needed. A booster or steam jet compressor venturi or evactor type is connected to the flash tank and to a water cooled condenser and a supply of steam is provided which when passed through the booster at high velocity picks 25 up'vapor which is evaporated in the flash tank,

compresses and transfers it to the condenser. As' the vacuum increases the boiling point of the water is lowered, so that some of the water is vaporized by the heat of the liquid, whereby the temperature of the remaining liquid is released.

-The higher the vacuum in the iiash tank the cooler the temperature of the liquid will be. For example, if the pressure in the flash tank is reduced to a value say 0.2575 inch of mercury ab- 35 solute pressure, the temperature of the water'inv the tank will be about 41 F. I

In systems of this type, one or more boosters or steam jet compressors are employed and are connected in multiple to a steam supply header. 'Ihe y number of boosters employed at any `one time is dependent upon the refrigerating load. Each booster or compressor may include one or a group of nozzles, and the steam in passing through them is converted from pressure head into velocity head which is high enough to overcome the back pressure in the condenser and to compress and transfer air and water vapor from tha flash tank to the condenser.

rPhe steam flow through a nozzle is theoretif cally a function ofv the absolute nozzle pressure while the vapor flow from the ilashtank to the condenser is a function of the difference between the pressure in the flash tank and in the condenser'. When this differential is at the proper value and the nozzle pressure is at its eiiicient operating point, any increase in steam pressure .above that required to maintain the proper pressure differential between the flash tank and the condenser will not effect any material increase l in the rate of transfer or vapor and air from the flash tank to the condenser, while the differential would tend to increase because of the increased load on the condenser.

Ifgtherefore, the steam pressure at the nozzles is regulated or controlled to bear a denite'relationship to the differential between the flash tank and condenser pressures, decided economies in the amount of steam and condenser cooling water employed may be realized.

An object of this invention is the provision of a regulating systemv whichwill respond to changes in the difference between the flash tank and condenser pressure, hereinafter called the pressure differential, and so control the pressure.

of the steam at the'nozzles that a denite relationship is maintained between the nozzle steam pressure and said pressure differential.

Another object is the provision of a regulating system which will respond to changes or varia-I tions in a given pressure differential to control the pressure of the steam at the nozzles in such fashion as to maintain a definite relationship between the nozzle steam pressure and the pressure differential regardless of the number of boosters or nozzles that may be operating.

Other objects of the invention will, in part, be apparent and will, in part, be obvious from the following description taken in conjunction with the accompanying drawing, in which:

Figure l is a more or less diagrammatic view of a stean refrigerating system provided with a control system embodying one form of the invention, parts of the apparatus of the systems being shown insection;

Fig. 2 is a. fragmentary view in section of a f portionof a regulator embodied in the control system;

Fig. 3 is a view in 'vertical section of an escapement valve associated with the regulator; and

Fig. 4 is afragmentary view in side elevation of the flash tank showing a plurality of steam jet boosters or ebmpressors connected to a common header in which the steam pressure is regulated by one regulator.

'I'hroughout the drawing and the `specification, like ref erence characters indicate like parts.

In Fig. 1 of the drawing, a flash tank or vacuum chamber I is illustrated in which the liquid, such as water, is refrigerated. Thewater or liquid to be cooled is introduced into the iiash tank by a pipe 2 having sprays 3 extending from opposite sides thereof and from which the water discharges into the tank. The cooled or chilled liquid or water is discharged into a cooling sys-.`

tem, not shown, by means of a pump 4. The water in the flash tank is chilled or cooled by maintaining a partial vacuum therein, the .degree of vacuum depending uponl the temperature of liquid desired. The vacuum is maintained by vmeans of one or'more steam jet compressors or boosters 5 connected to the top of the flash tank and to the top of a water cooled condenser 6. Cooling water for condensing the steam is delivered to the condenser through an inlet pipe I and is discharged therefrom through an outlet 8. The air and water vapor in the condenser is exhausted by means of a vacuum pump 9, and the condensate is discharged therefrom by means of a pump Iii'.

The steam jet compressors or boosters 5 are, as stated previously herein, operated by steam which is delivered thereto by means of a plurality of steam nozzles II. Each booster may have one or a. plurality of nozzles Il, which always function as a group, but the number of boosters in operation at any one time is dependent on the load and these may be turned on or olf manually or automatically in accordance with the load requirements. Steam is' supplied to the boosters from a supply pipe or header I2, and the pressure of the steam at the nozzles of these boosters is controlled by means of regulator I3. This regulator is illustrated as comprising a valve I4 and a pressure actuated device I5 for adjusting the valve in accordance with the value oi' steam pressure required.

The steam, in passing through the nozzles of the booster or boosters is converted from pressure head into velocity head, which creates a vacuum and lowers the pressure in the iiash tank and eifects the transfer of air and water vapor from the iiash tank to the condenser. If the temperature of the Water is to be maintained at 41 F., for example, then the absolute pressure in the iiash tank is maintained at about 0.2575 inch of mercury. The action of the steam jet compressors or boosters and the steam nozzles employed for creating and'maintaining the desired vacuum in the flash tank is well understood in this art and for that reason no further description thereof is believed necessary.

The control system of this invention is designed primarily to secure economy in the use I of steam by regulating the steam pressure in accordance with the pressure differential between the ilash tank and condenser. -The function of the evactor is to transfer water vapor and air from the ash tank to the condenser at such a rate as to maintain the proper degree of vacuum in the flash tank. In accomplishing this transfer, the evactor must work against a pressure head which is equal to the diiference, in

absolute pressure, between the flash tank and condenser pressures. The absolute pressure in' the flash tank will vary only with variations in the temperature of the chilled water, while the pressure in the condenser will vary with the amount of steam being condensed and the q'uantity of available cooling water and its temperature. r

If the condenser water is cooled by a cooling tower (not shown) considerable variations may be expected in the condenser pressure. When the outside wet and dry bulb conditions are high, the refrigeration load will be high and the cooling water for the condenser lwill be warm. Under these conditions the maximum pressure difference between the iiash tank and the condenser will exist. As the outside wet and dry bulb conditions become lower, the refrigeration requirements become less with the result that the cooling water for the condenser will be colder; Thus less pressure difference exists between the ash tank and the condenser under low outside wet and dry bulb conditions.

If the steam pressure at nozzles II is higher than that required to transfer the vapor and air from the iiash tank to the condenser at a rate which will maintain the required pressure difference between the ash tank and the condenser, no material increase in the amount of vapor transferred is eiected because of the characteristics of the booster or compressor, while excess heat is put into the cooling Water unnecessarily because of the excess steam employed. If the pressure of the steam at the nozzles were controlled in accordance with the pressure difierential existing between the flash tank and the condenser, considerable economies in the use of steam may be attained. Theoretically, the ow of steam through a nozzle is a function of the absolute nozzle pressure, While the flow of vapor through the booster is a function of the diierence in pressure between the condenser and the ash tank when the evactor is working at its most eicient point.

In accordance with this invention, control apparatus is provided which will maintain a definite relationship between the pressure diierential existing between the ash tank and condenser and the pressure oi' the steam at the nozzles, thereby avoiding the use of excess steam to accomplish the work involved. The capacity of the refrigerating system would be adjusted by changing the number of boosters or compressors employed at any one time either by hand or automatically, without in any way interfering with the operation of the control system in maintaining the above stated relationship between the nozzle steam pressure and the pressure differential between the flash tank and condenser.

The apparatus embodied in the control system comprises a regulator I 1 which is responsive to the pressure in the condenser and in the iiash tank and to the steam pressure at the-nozzles and causes regulating device I3 to operate in such fashion that the' steam pressure at the nozzles will be caused to bear a definite relationship to the differential between the flash tank and the condenser pressure.

The steam actuating device I5 comprises a pressure-tight housing I8 having therein a ilexible member I9, such as a bellows, which carries a thrust plate 20 at its upper end. Thethrust plate when it is in' its uppermost position engages spaced lugs 2I on the top of housing I8 so that actuating pressure when admitted to the housing will compress the bellows and move the thrust plate downwardly. A boss 22 centrally positioned on the lower side of plate 20 is coupled to stem 23 of valve I3 by means of a push rod 24 and adjustable couplings 25. These couplings provide a means for adjusting the valve in relation to the limits of travel of the bellows. The lower limit of travel of thrust plate 20 is xed by a tubular stop 26 which is threaded on boss 22 to provide adjustment in the downward movement of the plate. Movement of the bellows is resisted by a coll spring 21 located between the thrust plate and the bottom of housing I5. The coil spring urges the plate and valve stem upwardly and tends to actuate valve I4 to a position of minimum opening, in which case minimum steam pressure exists at nozzles II. As the pressure of actuating medium delivered to housing I 8 increases the thrust plate movesdownwardly thereby moving thel valve stem towards open vposition and increasing the pressure of the steam nozzles.

The operating medium for actuating device I5 may be compressed air taken from a supply pipe 28 in which the pressure is substantially constant, and transmitted thereto by -regulator I1 at varying pressures depending upon the relation between the nozzle steam pressure and the pressure differential between the flask tank and condenser.

Regulator I1 need not be of any special construction, as any regulator which will provide stability in the relationship between the nozzle steam pressure and the flash tank-condenser pressure differential will serve the purpose. The illustrated embodiment of regulator I1 is preferred, but as novel features of this regulator per se are disclosed and claimed in a copending application of Robert R. Donaldson, Jr., Serial No. 14,468 filed April 3, 1935, and assigned to John M.

Hopwood, no claim thereto is made herein.

Regulator l1 comprises an escapement valve 3D (see Fig. 3) which controls or determines, by its position in a valve body 3|, the pressure of the air delivered through a pipe 3|' to device I5;. a diaphragm 32 disposed in a housing 33 which responds to the difference in pressure between the flash tank and condenser; a steam pressure responsive device 34; and a lever 35 fulcrumed on a roller'36 and operatively connected, at one end and on one side of the fulcrum by a link 35 to device 34 and on the other side of the fulcrum to the pressure differential responsive diaphragm 32. Thus the escapement valve 30 is under the joint control of the pressure differential responsive diaphragm and to the steam pressure responsive device 34 so that the steam pressure at nozzles II will be controlled and adjusted in accordance with the pressure differential between the flash tank and' condenser.

As will be disclosed later herein,.the range of steam pressures which will balance a corresponding range of pressure differentials between the flash tank and condenser may be changed by ad justing the position of fulcrum 36 with respect to link 35.

In order that the weight of lever 35 may not affect the operation of the diaphragm 32 and device 34 in their responses to the flash tank and condenser pressure differential and the steam pressure, respectively, a relatively light spring 39 is connected at one end to lever 35 at a suitable point between fulcrum 36 and the connection between valve 30 and the diaphragm 32, and at its opposite end to an anchor 40 which is supported by an adjustment screw 4.I from a support housing 42 carried by a frame 43 on which valve 30 'and body 3| are supported. This spring is adjusted to carry the unbalanced weight of lever 35 and maintain it in operating engagement with the valve and diaphragm connection. The opposite end of the lever is connected to an adjustable tension spring 44 which fixes the loading that device 34 operates under and provides a means for initially adjusting the steam pressure at nozzles I I to the proper value for a given flash tank and condenser pressure differential. Regulator I1 is in equilibrium at these values of pressure differential and steam pressure, and thereafter the regulator will automatically adjust the steam pressure so that the regulator will always be in balance for different values of pressure differential.

Regulator I1 also embodies a stabilizing means in the form of a dash-pot 46 which acts to provide a positive but gradually diminishing gradient against which the pressure differential diaphragm 32 must work when moving in either direction,

that is, up or down as seen in the drawing. This gradient diminishes from the maximum value it exerts at the beginning of movement of the diaphragm to zero value and at a rate which allows suicient time `to elapse to effect the necessary;`

zles II in use is changed to suit a change in load on the refrigerating system. l

The dash-pot comprises a coil spring 41 which is connected at its upper end to the pressure differential responsive diaphragm and at its lower end to a piston 48 disposed in a vertically extending cylinder 49. The upper end of the cylinder is open to the atmosphere and the lower end is connected to a chamber 50 formed by a flexible member such as a bellows. The bellows is enclosed in a pressure tight housing I to which air pressureis .admitted by a pipe 52 at pressures determined and controlled by valve 30. The lower end of the) bellows is closed by a thrust plate 53 which in its lowermost position is spaced from the bottom of the housing by lugs 54 to allow the air pressure to work properly on the bellows. A compression spring 55 located inside the bellows yieldingly opposes contraction of the bellows and provides a positive gradient against which the air pressure must work.

Housing 5I is carried by a frame 56 which supports the diaphragm housing and the aforementioned parts above it, and the housing also car' ries a bracket 51 onl which a steam pressure responsive device 34 and the fulcrum 36 are supported. I

The space in cylinder 49 above and below piston 48 and the space within bellows 50 are lled with a non-compressible fluid such as pil and the bottom of the cylinder is connected by pipe 58 which is connected at its top with the interior of cylinder 49 at a point above the piston, at a height above the uppermost limit of travel of the piston. The pipe 58 is provided with a needle valve 59 that fixes the rate, for a given oil pressure, at which oil may ow from the space below the piston to the space above it, or -vice versa, in case either the bellows contracts or expands, respectively, or in case the piston is moved downwardly or upwardly, respectively, by the diaphragm 32.

If diaphragm 32 moves downwardly in re-I sponse to an increase in the pressure differential between the flash tank and the evactor, spring 41 is compressed and opposes such movement of the diaphragm but the spring compression tends to move the piston 48 downwardly at a rate fixed by the rate at which the oil can flow from the space below the piston to the needle valve to the space above it. As the piston moves downwardly the restraint offered by the spring tends to diminish but, while this restraint is diminishing, valve 30 is being opened to deliver air at increasing pressure to device I5 and to the interior of hous- .ing 5I, thereby increasing the steam pressureat the nozzles 1, because valve I4 has been opened wider, and increasing the pressure acting-on the dash-pot bellows 58. Bellows 50 therefore contracts an amount proportional to the pressure exerted on it and exerts an hydraulic pressure on the lower side of the piston 48 tending to move it upwardly and to further compress spring `41. This increased compression in spring 41 tends to move the diaphragm 32 upwardly, but this tendency diminishes at a rate proportional to the rate at y the nozzle steam which the oil iiows through the needle valve into the space above piston 48 in cylinder 49. This increased pressure on the dash-pot bellows and on the bellows of the steam responsive device 35, slows down the corrective action of diaphragm 32 sufliciently to permit enough time to elapse to effect the proper adjustment in the steam pressure as called for by the change in the pressure diiferential between the flash tank and the condenser. By the time the steam pressure has been adjusted to balance the pressure differential acting on the diaphragm of regulator I1, the effect of the dash-pot has diminished to the vanishing point and the coil spring 41 associated therewith will have assumed the position it normally occupies when equilibrium is reached between the differential acting on diaphragm 32 and the steam pressure acting on the 4bellows steam responsive device 34.

From the above it will be apparent that theA magnitude of the pressure impulses delivered to the steam pressure regulator I3 and to the dashpot bellows 50 is proportional to the change in the pressure differential acting on ydiaphragm I1 and/or the steam pressure acting o n'device 34. For this reason, the greater these changes, the shorter will the time of correction be as fixed by dash-pot 46, because the higher the pressure acting on the dash-pot bellows, the higher the rate of oil flow through needle valve 59 will be. This action of the dash-pot prevents over-correction of pressure.

The dash-pot working in conjunction with diaphragm 32 therefore gives stable operation to regulator I1 and avoids over or under adjustments of the steam pressure whenever a change in the pressure differential between the ash tank and the condenser occur.

In Fig. 2 of the drawing, the construction employed for providing an operating connection between diaphragm 32 and the valve 30 and between the diaphragm and the 'dash-pot coil spring 41, is illustrated. As shown, thrust plates 68 and 6I are disposed on the upper an lower sides of the diaphragm and give support and rigidity to the central or middle portion thereof. The center of the diaphragm and the plates are apertured to accommodate a connecting rod 62, the ends of which extend upwardly and downwardly, respectively, through the diaphragm housing. The plates 60 and 6I are held flrmly against the diaphragm by means of bushings 63 and nuts 64.

In order to provide a pressure tight joint where rod 62 passes through housing 33, suitable flexible diaphragms 65 are secured between the bushings 63 and Washers 66, the required pressure being supplied by nuts 64, and by pressure tight sealing rings 61 which seal the outer edges of the openings through which rod 62 passes.

The lower end of rod 62 is provided with an anchor block 68 around which the upper end of coil spring 41 is disposed, and to which it is selcurely clamped by means of a clamping ring 69 and a screw 18.

The upper end of rod 62 is operatively connected to valve 30 and lever 35 by means of a yoke 1I which is secured between the upper -nut 64 and washer 66. The upper end of the yoke is provided with a flange 12 which overhangs lever 35 and which carries a knife edge 13 on its underside which is engaged by lever 35. The upperside of this ilange carries a pin 14 having a conical point which seats in a socket 15 in the lower end of a push rod 11 that engages the lower end of valve 38. see Fig. 3.

When diaphragm 32 moves upwardly, it moves valve 30 upwardly to a position which reduces the area of opening 18 in the inlet portof valve body 3l and increases the area of opening 19 in the exhaust port at the lower end of the valve body through which air may exhaust to the atmosphere. Thus, the relative areas of the inlet and exhaust openings fixes the pressure of the air existing within valve body 3| and as transmitted through outlet port to pipe 3 I which communicates with the interior of the housing I8 device I5 and to the interior of the dash-pot bellows housing 5|. A

Valve 38 as shown in'Fig. 3, is yieldingly urged towards open position, that is, towards maximum pressure transmitting position by means of a coil spring 82. Therefore, as the diaphragm moves downwardly, this spring will move valve 38 downwardly and cause it to follow push rod 11.

The exhaust opening 19 is lformed in a bushing 83 which is threaded into the lower end. of the valve body and provides an adjustment which fixes the amount of travel required to move valve 30 upwardly to its extreme closed position or downwardly to its extreme open position, thatA is, tothe position which the exhaust port opening 19 is closed.

In case it is desired to change the values of steam pressure required by device 35 to balance a given differential acting on diaphragm 32, the fulcrum roller 36 may be shifted longitudinally of lever 35 either towards or away from spring 39. The closer fulcrum 36 is to the connection of the steam pressure bellows to lever 35, the higher the steam pressure must be to balance a given pressure diierential between the ash tank and the condenser, and the farther -it is away from this point of connection the lower the steam pressure must be to balance this differential.

To provide for this adjustment fulcrum roller 36 is mounted on a carriage 84 which is slidable on bracket 51. The adjustment of the carriage may be accomplished by means of a screw which is threaded into the fulcrum carriage and journalled in a bearing 86 which cooperates with collars 81 on the screw to prevent its longitudinal movement. By turning a handwheel 88 fastened to this screw, the fulcrum carriage and its fulcrum roller may be shifted longitudinally 0f lever 35 to change the relation between the steam pressure acting on the nozzles and the pressure differential between the flash tank and the condenser.

The operation of the control system above described is as follows: Assume that the absolute pressure to be maintained in ilash tank I is P1 pounds per sq. in. and that the absolute pressure in the condenser is P2 pounds per sq. in., then the pressure differential in pounds per sq. in. is equal to (Pif-P2). Assume also that for this differential of (P1-P2) that the evactor or venturi 5 requires a steam pressure on the nozzles II of P pounds per square inch. With these assumed conditions, regulator I1 is then adjusted, as hereinbefore described, until a steam pres- If all of the steam boosters are not operating and another booster is turned on therel will be a pressure drop in the steam pressure acting on the nozzles of these boosters because of the increased steam flow through valve I4. This reduced steam pressure is reflected in the steam pressure responsive device 35 with the result that regulator I1 is unbalanced. When this happens the diaphragm 'moves downwardly to open valve 30 an amount depending upon the unbalance and increases the pressure of the air in device I5. This results in a further opening of valve Il and increased steam pressure. 'I'he increased steam pressure results in increased steam iiow lthrough the booster nozzles so that the pressure in the condenser is increased thereby increasing the difference between the pressure in the iiash tank and condenser. This increase in differential further unbalances regulator II in a direction which will further increase the air pressure in device I and further increase theextent of opening of the steam valve I4. This action continues until the steam pressure acting on the nozzles of the boosters-is in balance with the pressure diEerential (P1-P2) acting on diaphragm 32. At this new equilibrium between steam pressure and pressure differential, regulator II will maintain a higher steam pressure on the booster nozzles but the value of Pl-P2 will be the same for any value of steam pressure required to maintain it. Regulator I1 acts to regulate the steam pressure directly in accordance with the diierential in pressure between the ilash tank and the condenser independently of the number of boosters in use at any particular time. And regardless of the number of boosters in use, the steam pressure acting on nozzles II will be regulated tomainrtain the value of (Pl-P2) constant even though the individual values of P1 or P2 may vary slightly. But since regulation of the steam pressure results from a change in either P1 or P2 or vin the steam pressure and since regulator I'I is initially adjusted to maintain the difference betweenPl and P2 constant, it will be apparent that the steam pressure will always be adjusted to maintain this difference whenever P1 or P2 or both change in values to depart from a constant diierence between them.

While but one embodiment'of the control system or this invention has been shown and described, it will be apparent to those skilled in this art that various modifications and changes may A be made in thesystem and in the operative elements of the system without departing either from the spirit or the scope of the invention.

- What I claim as new and desire to secure by Letters Patent is:

1. In a steam reirigerating system having a vacuum chamber in which iiuid is cooled, a condenser, and a steam :let compressor connected between said chamber and condenser, a source of supply of steam connected to flow .through said compressor to establish a partial vacuum in the chamber, means for varying thepressure of steam delivered to said compressor, and a source or supply of cooling iiuid for the condenser for condensingrthe steam delivered thereto, the combination of a device having means responsive to the pressure of the steam as delivered to said compressor and means responsive to the pressure in the vacuum chamber and condenser, for so controlling the steam pressure varying means that the pressure of the steam as delivered to said compressor is varied in accordance with variations in the pressures existing in the vacuum chamber and the condenser.

2. In a steam reirigerating system having a vacuum chamber in which iiuid is cooled, a condenser, and a steam jet compressor connected between said chamber and condenser, a steam nozzle disposed to deliver steam at' high velocity through said compressor to establish a partial denite relationship to the diilerential in pressure between the flash tank pressure and the condenser pressure.

3. I n a' steam refrigerating system having a flash tank in which uid is cooledby the action oi' partial evaporation, a condenser, and a booster connected between said tank and condenser, a steam nozzle disposed to deliver steam through said booster at high velocity to establish avacuum in said flash tank, means for controlling the nozzle steam pressure, means responsive to flash tank pressure and condenser pressure', means responsive to the nozzle steam pressure, and means jointly controlled by all said pressure responsive means for operating the nozzle steam pressure controlling means to maintain a predetermined relationship between the nozzle steam pressure f and the diierence between the flash tank and condenser pressures.

4. In a steam refrigerating system having a ilash tank in which iiuid is cooled by the action of partial evaporation, a condenser, and a plurality of steam jet compressors connected between sald tank and condenser, each compressor having a plurality of steam nozzles disposedto deliver steam through said compressors at high velocity to establish a vacuum in said ash tank, means for controlling the steam pressure at the nozzles, means responsive to the ilash tank pressure and the condenser pressure, means responsive to the nozzle steam pressure, omeans jointly controlled by` all said pressure responsive means for maintaining a predetermined ratio between thev nozzle pressure and the difference between the iiash tank and condenser pressures ,independently ci.' the number of compressors in operation, and means for adjusting the ratio of steam pressure at the nozzles to the pressure differential between the flash tank and condenser.

5. In a steam refrigerating system having a ilash tank in which iiuid is cooled by the action of partial evaporation, a condenser, and a booster connected between said tank and condenser, .a steam nozzle disposed to deliver steam through said booster at high velocity to establish a vacuum in said ilash tank, means for controlling the nozzle steam pressure, means responsive to flash tank pressure and condenser pressure, and

vmeans controlled by said pressure responsive means for operating the nozzle Asteam pressure controlling means to maintain a predetermined relationship between the nozzle steam pressure and the diierence between the ilash tank and condenser pressures.

- AIFRED A. MARKSON. 

